Doesn't the Mamba have only USB input? You'd have to get the D10, D12 or DB1 first; I don't know of any portable device that can output digital signal via USB (they're usually just for file transfers and charging.) Then you can take a look at, say, the upcoming Fiio X3.

The D4 Mamba has both USB and headphone/RCA inputs. It's a great headphone DAC/amp.

TS was was asking about using it with a portable transport as a complete DAC-Amp. So far only phones and tablets stream audio via USB - old iRivers and the upcoming X3 only have SPDIF. The latter though at least uses the same DAC chip so it's very likely (same chip doesn't mean identical SQ) he'll like the as much as the D4's.